Measurements have been made of the third-order nonlinear-optical susceptibilities of gaseous infrared saturable absorbers. These coefficients are relevant to mode locking of infrared lasers such as CO2. The measurements are in good agreement with a theory based on a four-level model of the absorber. Earlier mode-locking observations are interpreted in light of the new data. The value of the transition moment and some other parameters of SF6are deduced from the measurements. For SF6at torr, the third-order nonlinear coefficient is in the range C.m/V3, when interacting waves differ in frequency by less than the molecular collision frequency. We find that the average dipole transition moment for SF6near the or lines of CO2is approximately 0.1-0.2 Debye. The nonlinear coefficient of CH3F is typically three orders of magnitude smaller than that of SF6.